The invention relates to a multistage process for continuously preparing a polyamide 6 with a low dimer content from .epsilon.-caprolactam and/or from recycled lactams (so-called return lactams) which are obtained in extraction stages and melt demonomerization stages for reducing the low molecular mass fraction in the case of polyamide 6.
It is known that during the polycondensation of .epsilon.-caprolactam to give polyamide 6 a chemical equilibrium is established which is temperature-dependent. At a common plant reaction temperature of 280.degree. C., a water-soluble fraction of around 13% is established.
For the further processing of the polymer it is necessary to dissolve out the unreacted fraction. Various methods are known for this purpose. For example, the melt in the equilibrium state is used to prepare granules which are extracted with water. In that case .epsilon.-caprolactam, oligomers and dimers are dissolved out. This method necessitates subsequent drying of the granules and processing of the aqueous extraction waters by evaporative concentration.
Also known are methods in which unreacted .epsilon.-caprolactam, dimers and oligomers are driven off from melt films by means of reduced pressure or inert gas.
For economic reasons, these recovered products should be supplied to the polymerization process again.
There are methods in which PA 6 extract water concentrates with high or low solids contents are added in smaller or greater proportions to the fresh lactam in order that they too may be reacted to give polyamide 6. In accordance with DE-B 25 01 348 and DE-A 27 32 328 polyamide 6 extract waters concentrated to a solids content of more than 90% or, respectively, to 60-70% are polymerized with more than the equal amount of fresh lactam, based on the solids content, with propionic or, respectively, benzoic acid in a reactor which is operated essentially at atmospheric pressure.
A disadvantage of this process is that when polyamide 6 extract water concentrates are polymerized with monocarboxylic acids, the polymerization times required are longer than in the case of polymers of fresh lactam with monocarboxylic acids. In addition, the conversions of .epsilon.-caprolactam or cyclic oligomers obtained are from about 2 to 3% below those of fresh lactam polymers, since the cyclic oligomers take essentially no part in the reaction mechanism. The high water content in the reaction mixture, in conjunction with the pressureless operation of the polymerization reactor, leads to an unfavorable reaction course. The water is unable to act in the reaction because it evaporates.
It is also known that melt vacuum demonomerization processes are operated such that the demonomerization products drawn off in gas form, i.e. .epsilon.-caprolactam and cyclic oligomers, are condensed in fresh lactam and so also subjected to the polymerization (DE-A 28 21 886 and DE-A 29 48 865). In the case of similar processes as well, the cyclic oligomers, especially the cyclic dimers, are not converted into reactive compounds.
Also known is the strong catalytic action of o-phosphoric acid on the polymerization of .epsilon.-caprolactam [Geleji, F. et al. "Wirkungsmechanismus verschiedener Katalysatoren bei der Caprolactampolymerisation" [Mechanism of action of various catalysts in the polymerization of caprolactam] in Faserforschung und Textiltechnik 13 (1962) 6, 282-283] in terms of the polymerization of cyclic oligomers (Schade, H. and Beckhaus, F. W.: Verfahren zum Polymerisieren von Oligomeren des .epsilon.-Caprolactams [Process for polymerizing oligomers of .epsilon.-caprolactam]--DD Patent 24 307). Disadvantages of such a polymerization technology that occur are, inter alia:
excessively rapid increase in viscosity and hence inadequate reproducibility of the desired degree of polymerization, since the o-phosphoric acid acts essentially as catalyst and to a lesser extent as chain regulator PA1 inadequate conversion to .epsilon.-caprolactam and cyclic oligomers, and PA1 depolymerization processes when polyamide 6 granules of this kind are processed.
In order to avoid the abovementioned problems it is common to separate off the oligomers when processing the aqueous extraction waters by distillation. The oligomers separated off are landfilled, which pollutes the environment, or undergo energy-intensive depolymerization into lactam, by processing with phosphoric acid, and the lactam is subsequently passed to a cleaning stage. The residues are land-filled.
For technical reasons concerned with the apparatus and on economic grounds, polyamide manufacturers have to date concentrated the lactam oligomer mixture to a residual water content of .ltoreq.2% by evaporative concentration. With this water content it becomes impossible to bind the cyclic dimer chemically into the polymer structure.
The aim of the invention is to restrict as much as possible the formation of dimers and oligomers in the preparation of polyamide 6 from fresh lactam and to bind cyclic dimers, especially cyclic dimers introduced by way of recycled lactam, and also cyclic oligomers into the production process and to obtain the processing of the total amount to give high-grade end products while creating a closed circuit of materials (no waste).